Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic elements written at least in part by a current driven through the magnetic element.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional pinned layer 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. Also shown is top contact 24.
Conventional contacts 11 and 24 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator such as MgO. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. Direct exposure of the conventional free layer 20 to the top contact 24 may result in a disordered interface, dead magnetic regions and enhanced damping. Consequently, the conventional capping layer 22 is provided directly on the free layer 20, prior to deposition of the top contact 24. This conventional cap acts as a diffusion block and improves the surface quality of the conventional free layer 24. The conventional capping layer 22 is typically made of materials such as Ta.
The conventional pinned layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional pinned layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 14. Although depicted as a simple (single) layer, the conventional pinned layer 16 may include multiple layers. For example, the conventional pinned layer 16 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. Further, other versions of the conventional MTJ 10 might include an additional pinned layer (not shown) separated from the free layer 20 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru.
Spin transfer torque may be used to write to the conventional MTJ 10. In particular, spin transfer torque rotates the magnetization 21 of the conventional free layer 20 to one of the two directions along its easy axis. When a write current is passed through the conventional MTJ 10 perpendicular to the plane of the layers, electrons may be spin polarized by transmission through or reflection from the conventional pinned layer 16. The spin transfer torque on the magnetization 21 of the conventional free layer 20 may be adequate to switch the conventional free layer 20 if a sufficient current is driven through the conventional MTJ 10. Therefore, the conventional free layer 20 may be written to the desired state. The conventional MTJ 10 may thus be used for data storage in an STT-RAM.
The conventional MTJ 10 is required to be thermally stable for use in STT-RAM. During periods of latency, when the conventional MTJ 10 is preserving a previously stored datum, the magnetization 21 of the conventional free layer 20 is not completely static. Instead, thermal fluctuations allow the magnetic moments within the conventional free layer 20 to oscillate and/or precess. The random nature of these fluctuations translates to the occurrence of generally rare, unusually large fluctuations. These fluctuations may result in the reversal of the magnetization 21 of the conventional free layer 20, making the conventional MTJ 10 unstable. The probability of such a reversal decreases with increases in the height of the energy barrier between the two stable orientations (along the x-axis as shown in FIG. 1) of the free layer magnetization 21. Thus, for a memory employing the conventional MTJ 10 to be thermally stable, the conventional MTJ 10 should have a sufficiently high energy barrier that the magnetization 21 of the conventional free layer 20 is not switched by such thermal fluctuations. This energy barrier is typically achieved through a magnetic anisotropy energy sufficient to retain the magnetization 21 in the direction it was written. This magnetic anisotropy of the free layer 20 is generally large, in plane and along a particular axis. For example, in the conventional MTJ 10 shown in FIG. 1, the anisotropy would be along a horizontal (easy) axis, allowing the free layer magnetization 21 to be stable when the magnetization 21 is stable along the x-axis in FIG. 1.
Although a large energy barrier is desired for thermal stability, a large energy barrier may adversely affect writeability of the conventional MTJ. In general, a larger energy barrier provided by the in plane, generally uniaxial anisotropy results in a larger switching current. A larger write current would be driven through the conventional MTJ to switch the magnetization 21 of the conventional free layer 20. Use of a larger write current is generally undesirable for a variety of reasons including, but not limited to, increased heat generated and the potential need for a larger transistor in a magnetic memory cell. Thus, thermal stability may be considered to be at odds with a reduced write current.
Accordingly, what is needed is a method and system that may improve the thermal stability of the spin transfer torque based memories. The method and system address such a need.